1. Field of the Invention
The present invention relates to a multilayer panel comprising a core layer A having a honeycomb structure or an open-celled foam structure and at least one cover layer C made of a fiber-reinforced thermoplastic polymer, wherein said layers A and C are bonded to each other by an adhesive B which is a thermoplastic polymer.
Owing to new fire protection requirements governing the interior trim of aircraft, there is an increasing demand for high-strength, high stiffness, multilayer panels having polymeric matrix.
2. Description of the Related Art
Multilayer panels composed of a core layer having a honeycomb structure and cover layers made of thermoset polymers are known. The bond between the layers is customarily due to a liquid adhesive which is applied to the cover layers or due to an adhesive film. In either case the adhesive is preferably an epoxy resin with or without slight incipient polymerization.
DE-A-2 009 090 (U.S. Pat. No. 3,656,992) recommends that core layers having a cell structure and porous cover sheets should be bonded together using perforated adhesive films which preferably consist of a heat-curable epoxy resin which may contain a thermoplastic polymer in admixture. As the resin is heated and cured, bulges of adhesive develop at the points of contact between the cover sheets and the edges of the cell structure cores, which is said to bring about good adhesion.
However, multilayer panels based on thermosets do not have a sufficiently low flammability rating for many purposes, in particular for aircraft interiors. In addition, the strength and stiffness and the moisture gain of prior art multilayer panels leaves something to be desired.